Gas mixtures of hydrogen, oxygen, water vapor and a host gas, such as nitrogen, commonly exist in nuclear plant containments in varying concentrations, depending upon the design and operational characteristics of the plant. The atmosphere in the containment is usually monitored for H.sub.2 and O.sub.2 content to assure that explosive mixtures do not arise that might threaten the integrity of the containment. At the present time, means for sensing these gases in the presence of dry nitrogen is provided by electrochemical cells whose active element (the electrolite) is chosen to be sensitive to either H.sub.2 or O.sub.2, but not both, and to be inert to nitrogen.
Electrochemical cells of the above-mentioned type have limited operating lives and are difficult to qualify for use in the post-accident environment of the containment. Their sensitivity is affected by radiation fields, excessive water vapor, and a variety of contaminants usually found in the containment itself. Complicated and expensive sampling systems are required for their use.
During normal reactor operations, H.sub.2 and O.sub.2 are produced by electrolysis of water. Tritium is also produced in smaller quantities. Normally, these gases are dissolved in the primary coolant but can accumulate in the containment and the radwaste building over long periods of time. Hydrogen recombiners are provided to handle this source.
Reactor transients can lead to depressurization of the primary system and evolution of the dissolved gases. In the unlikely event that the fuel temperature exceeds about 1500.degree. C., the zirconium fuel cladding can be oxidized by water vapor, releasing large quantities of H.sub.2. Certain oxides of zirconium are unstable and can release O.sub.2 to the containment atmosphere. Thus, the relative H.sub.2 /O.sub.2 concentrations can vary widely and possibly reach explosive proportions-above 5 v/o (volume percent) in dry air. The explosive mixture composition is appreciably dependent on water vapor content. Therefore, accurate assessement of hazardous conditions requires knowledge of the local relative humidity as well as the H.sub.2 /O.sub.2 ratio.
Means for handling large and variable H.sub.2 /O.sub.2 concentrations rely heavily on the type of sensing method used. Accuracy and reliability are very important, as are insensitivity to radiation and contamination. In view of the drawbacks of prior art techniques, a need has arisen for an improved method of sensing H.sub.2, O.sub.2, and water vapor in gas samples, especially those taken from a nuclear plant containment for monitoring purposes.
Prior U.S. patents relating to the detection of gases include the following:
______________________________________ 3,429,177 3,468,157 3,724,484 3,805,590 3,902,365 3,977,394 3,981,176 4,119,950 4,155,246 4,220,040 4,235,099 4,236,827 4,246,773 ______________________________________
U.S. Pat. No. 3,429,177 discloses a method for detecting hydrogen gas using two acoustic waves. Hydrogen can be detected by sensing a change in the velocity between the acoustic waves.
U.S. Pat. No. 3,724,484 discloses a gas detector used in nuclear reactor environments to analyze the density of nuclear particles in hydrogen gas that flows into a reactor cavity. U.S. Pat. No. 3,977,394 is directed to the use of a computer for gas analysis and discloses the use of a spirometer to produce a signal proportional to the volume of air inhaled and exhaled by a person.
U.S. Pat. No. 3,902,365 discloses the use of a tuning fork driven at its natural frequency by a regenerative piezo electric drive; U.S. Pat. No. 4,235,099 discloses a method for measuring the density of a liquid using ultrasonic waves; U.S. Pat. No. 4,236,827 discloses an opto-acoustic gas analyzer using a black body light source and a pressure detector for detecting pressure changes in a gas; and U.S. Pat. No. 3,805,590 discloses a sensor for the partial pressure of oxygen using ultrasonic waves. The remaining patents relate to the art of ultrasonic gas analysis.